Gaining the system: limits to compensating color deficiencies through post-receptoral gain changes

Color percepts of anomalous trichromats are often more similar to normal trichromats than predicted from their receptor spectral sensitivities, suggesting that post-receptoral mechanisms can compensate for chromatic losses. The basis for these adjustments and the extent to which they could discount the deficiency are poorly understood. We modeled the patterns of compensation that might result from increasing the gains in post-receptoral neurons to offset their weakened inputs. Individual neurons and the population responses jointly encode luminance and chromatic signals. As a result, they cannot independently adjust for a change in the chromatic inputs, predicting only partial recovery of the chromatic responses and increased responses to achromatic contrast. These analyses constrain the potential sites and mechanisms of compensation for a color loss and characterize the utility and limits of neural gain changes for calibrating color vision.